Generally, a problem inherent in monitoring electromagnetic radiation which is reflected from a sample outer surface is that it often is contaminated with reflections from a backside thereof. One approach to preventing this problem is to roughen the backside. Another is to place a mask on the surface of the sample which allows surface reflections to proceed, but prevents backside reflections from exiting. Another approach yet is to place a blocking means between the sample and the detector which blocks backside reflections but lets front surface reflections proceed into the detector. Said approaches, however, do not allow for selectively monitoring electromagnetic radiation reflected from the front or backside. In that light it is noted that backside reflected electromagnetic energy contains information which is different from that contained in outer surface reflections.
Continuing, it is also disclosed that ellipsometry monitors the product of refractive index and thickness in samples investigated thereby, and that said parameters are correlated in what can be termed an optical thickness. A known approach to breaking the correlation is to obtain data from two samples comprising the same material(s) but which samples are of different thicknesses. Simultaneously regression of the data obtained from both sides onto corresponding mathematical models leads to substantially uncorrelated evaluation of refractive index and thicknesses.
It is also mentioned that it is known to obtain data from front and backsides of a sample and simultaneously regress said data onto a mathematical model therefore.
More on-point as regards the present invention, it is known to cause a polarized electromagnetic beam to impinge on the top surface of a sample along with monitoring reflected electromagnetic radiation therefrom to enable characterization of the sample as a composite. The detected reflected electromagnetic radiation generally includes interfering components which reflect from said top surface and from various interfaces between a plurality of layers.
It is also known to focus electromagnetic radiation onto a surface of a sample (e.g. 25 micron spot size), comprised of a front and back, and that electromagnetic radiation reflected from the front and back can become spatially separated from one another when such focusing is utilized. This avoids the complication of interference effects between various components reflected from different interface locations in the sample, by allowing separate monitoring of the various reflections. That is, the electromagnetic radiation reflected from a coating on the bottom of a sample can be monitored substantially free of components reflected from the top surface and various intervening interfaces.
further, it is known that appropriate orientation of an impinging beam of electromagnetic with respect to a curved sample surface can make the curved surface appear essentially as a flat surface, (e.g. align the beam orientation with a longitudinal dimension of a cylinder), to an electromagnetic beam of small diameter.
In addition, a Co-Pending patent application Ser. No. 11/288,785 it is disclosed to use a detector to selectively intercept portions of a focused beam caused to impinge on a sample comprising a top surface and at least one interface therebelow.
Known Patents and Published Applications are:                Patent Application No. 2002/0113200 A1 was identified as an aperture 103A is disclosed which can be placed near a detector to block entry of one of two beams from different sources.        U.S. Pat. No. 3,799,679 to Simko is disclosed as an iris (38) is present near a detector which can be adjusted to block entry of backside reflection thereinto.        Patents to Meeks, U.S. Pat. Nos. 6,130,749, 6,198,533 and 6,392,749 are disclosed for the presence of a hole 2022 in an integrating sphere near, but not atop a sample.        U.S. Pat. No. 6,088,092 to Chen et al. is disclosed as it applies a spatial filter (28) to block backside reflection entry into a detector.        U.S. Pat. No. 6,088,104 to Peterson is disclosed as a blocking element (B) is present which can be used to block electromagnetic radiation entry to a detector.        U.S. Pat. No. 6,097,482 to Smith et al. is disclosed as it applies baffles to block light entry to a detector.        U.S. Pat. No. 6,166,808 to Greve is disclosed as it describes use of an aperture near a detector to block backside reflections entry to a detector.        U.S. Pat. No. 5,936,734 is identified as it describes a method of partitioning electromagnetic radiation into coherent and incoherent portions when calculating intensity.        U.S. Pat. No. 6,455,853 to Herzinger at al. is identified as it describes obtaining data from both sides of a sample.        
The various known factors recited above however, have not, to the Applicant's knowledge, been combined to provide a non-destructive method of characterizing a thin film coating present on the inner surface of a tube shaped sample. Need remains for a system and non-destructive method which allows investigation of a film present on an inner surface of a tube shaped sample.